Download Paleoecological evidence for abrupt cold reversals during peak

Quaternary Research 71 (2009) 142–149
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Quaternary Research
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / y q r e s
Paleoecological evidence for abrupt cold reversals during peak Holocene warmth on
Baffin Island, Arctic Canada
Yarrow Axford a,⁎, Jason P. Briner b, Gifford H. Miller a, Donna R. Francis c
a
b
c
Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, UCB 450, Boulder, CO, 80309, USA
Department of Geology, University at Buffalo, 876 Natural Sciences Complex, Buffalo, NY, 14260, USA
Department of Geosciences, University of Massachusetts, 233 Morrill Science Center, Amherst, MA, 01003, USA
a r t i c l e
i n f o
Article history:
Received 1 August 2007
Available online 25 November 2008
Keywords:
Chironomidae
Holocene
Arctic
Midges
Paleoclimate
Paleolimnology
Abrupt climate change
a b s t r a c t
A continuous record of insect (Chironomidae) remains preserved in lake sediments is used to infer
temperature changes at a small lake in Arctic Canada through the Holocene. Early Holocene summers at the
study site were characterized by more thermophilous assemblages and warmer inferred temperatures than
today, presumably in response to the positive anomaly in Northern Hemisphere summer insolation. Peak
early Holocene warmth was interrupted by two cold reversals between 9.5 and 8 cal ka BP, during which
multiple cold-stenothermous chironomid taxa appeared in the lake. The earlier reversal appears to correlate
with widespread climate anomalies around 9.2 cal ka BP; the age of the younger reversal is equivocal but it
may correlate with the 8.2 cal ka BP cold event documented elsewhere. Widespread, abrupt climate shifts in
the early Holocene illustrate the susceptibility of the climate system to perturbations, even during periods of
enhanced warmth in the Northern Hemisphere.
© 2008 University of Washington. All rights reserved.
Introduction
Although the climate of the Holocene has been more stable than
that of the preceding glacial period, there is mounting evidence for
high-amplitude climate variations during the Holocene, particularly in
the North Atlantic region (e.g., Mayewski et al., 2004; Alley and
Ágústdóttir, 2005). Large Holocene climate changes imply undiscovered climate forcings and/or greater than expected sensitivity to
known forcings (e.g., due to positive feedbacks in the climate system).
Superimposed over known changes in solar insolation through the
Holocene are a number of less understood climate forcings, including
volcanic eruptions and solar variability, and perturbations such as
freshwater outbursts to the North Atlantic Ocean (e.g., Schmidt et al.,
2004). Estimates of climate sensitivity and feedbacks are a major
source of uncertainty in projections of future climate change (Knutti
et al., 2002; Stainforth et al., 2005; IPCC, 2007). Paleoclimate data
provide our only empirical observations of long-term climatic
responses to natural forcings, and therefore make important contributions to improving climate projections.
The eastern Canadian Arctic occupies a pivotal location for
paleoclimate reconstruction: Baffin Island (Fig. 1) is 500 km west of
Greenland; thus paleoclimate records from Baffin Island provide a
regional context for Greenland records and clarify how climate events
recorded in central Greenland ice cores affected nearby land masses.
This high-latitude region, like the rest of the Arctic, may be especially
⁎ Corresponding author. Fax: +1 303 492 6388.
E-mail address: [email protected] (Y. Axford).
0033-5894/$ – see front matter © 2008 University of Washington. All rights reserved.
doi:10.1016/j.yqres.2008.09.006
sensitive to past and future changes in climate forcing. Increased
radiative forcing is amplified in the Arctic by positive feedbacks, and
the Arctic in turn plays an important role in Earth's energy budget
(Holland and Bitz, 2003; Chapin et al., 2005; Serreze and Francis,
2006). Instrumental records indicate that the Arctic has warmed faster
than the global average over the past century, and model simulations
predict enhanced future warming at high northern latitudes (Serreze
et al., 2000; ACIA, 2005; IPCC, 2007). Paleoclimate data provide a basis
for constraining the magnitude of this amplified response to radiative
forcing, and means for testing whether polar amplification is indeed a
persistent feature of arctic climate.
This study reconstructs Holocene summer temperatures at Lake
CF8 on Baffin Island (Fig. 1) using subfossil insect (chironomid or nonbiting midge; Diptera: Chironomidae) assemblages as a proxy for
summer air temperatures. Our results add to the small database of
records that quantify temperatures throughout the Holocene in the
North American Arctic (e.g., reviewed by Kaufman et al., 2004; Kaplan
and Wolfe, 2006), and to our knowledge this is the first study in the
eastern Canadian Arctic to reconstruct terrestrial temperatures at
adequate temporal resolution to identify abrupt early Holocene
events.
Materials and methods
Lake CF8 (informal name) is a small, shallow lake (0.3 km2,
depthmax = 10 m) situated on an inter-fjord lowland on northeastern
Baffin Island in Nunavut, Arctic Canada, at 70° 33.37'N, 68° 57.14'W
(Fig. 1). The lake sits at 195 m asl and was overridden during the last
Y. Axford et al. / Quaternary Research 71 (2009) 142–149
143
Figure 1. (a) Overview map of the North Atlantic region, showing locations of the study site (Lake CF8); the Agassiz Ice Cap; NGRIP, GISP2 and DYE-3 ice cores; and lake Ammersee;
and (b) topographic map of the landscape surrounding Lake CF8 on northeastern Baffin Island.
glacial cycle by cold-based portions of the Laurentide ice sheet (Briner
et al., 2005; 2007a). Deglaciation of the lowland was complete by
∼ 12 cal ka BP (Briner et al., 2005; 2007b). The lake has no permanent
inflow stream and presumably receives surface water mainly from
snowmelt. At the nearby village of Clyde River, modern mean annual
temperature is −12.8°C and mean July temperature is + 4.4°C (Environment Canada, 2007).
A single-drive sediment core (02-CF8-01) containing 120 cm of
Holocene gyttja underlain by sand was recovered from the deepest
point in Lake CF8 using a percussion corer. Fifteen AMS 14C ages (Table
1; Fig. 2) were obtained on aquatic bryophyte macrofossils, which are
known to equilibrate with atmospheric CO2 in similar crystalline
terrains of Baffin Island (Wolfe et al., 2004). All ages were calibrated
using Calib v 5.0.2 (Stuiver et al., 2005) and the IntCal04 calibration
curve (Reimer et al., 2004). The age model used for this study is a
polynomial fit to all but the two deepest ages, which are inverted
(Fig. 2). The inverted ages make the age model relatively uncertain
below 107.5-cm depth (10.5 cal ka BP), where two different macrofossils yielded statistically overlapping ages. 95% confidence intervals
on the polynomial regression overlap with the 2σ calibrated age ranges
of all but one of the ages used in the model (Fig. 2) and provide
estimates of uncertainty for age assignments throughout the record.
Chironomid larvae are aquatic, and the larval head capsules are
chitinous and generally well preserved in lake sediments. Chironomid
assemblages are very sensitive to temperature change, and subfossil
chironomids have been used to reconstruct both abrupt and lowamplitude temperature changes (e.g., Walker et al., 1991b; Brooks and
Birks, 2000a,b; Cwynar and Spear, 2001; Larocque and Hall, 2003;
Brooks and Birks, 2004; Heiri et al., 2004; Caseldine et al., 2006;
Axford et al., 2007, 2008; Porinchu et al., 2007), provided that changes
in other environmental factors (e.g., lake depth or chemistry) over
Table 1
Radiocarbon ages from core 02-CF8-01, Baffin Island
Lab numbera
Depth (cm)
δ 13C
Fraction modern
14C age
(14C yr BP)
Calibrated age
(cal yr BP)b
AA60640
AA60641
AA60642
CURL-8927
CURL-8427
CURL-8428
AA60643
CURL-8929
CURL-8429
AA60644
CURL-8896
CURL-6954
CURL-8271
CURL-8142
AA60645
2
31
50
58
66.5
70.5
71
77
84
88
98
107
107.5
113
121
−24.8
−26.6
−29.1
−29.2
−24.7
−21.7
−24.2
−24.0
−24.6
−29.0
−27.8
−30.5
−21.4
−16.2
−26.5
0.9111 ± 0.0040
0.6766 ± 0.0052
0.5596 ± 0.0028
0.4677 ± 0.0009
0.3937 ± 0.0009
0.3962 ± 0.0009
0.3996 ± 0.0034
0.3836 ± 0.0012
0.3515 ± 0.0008
0.3530 ± 0.0022
0.3215 ± 0.0008
0.3133 ± 0.0017
0.3119 ± 0.0011
0.2794 ± 0.0009
0.3457 ± 0.0037
748 ± 35
3138 ± 62
4664 ± 41
6105 ± 20
7490 ± 20
7440 ± 20
7368 ± 68
7695 ± 30
8400 ± 20
8365 ± 50
9115 ± 25
9320 ± 45
9360 ± 30
10245 ± 30
8532 ± 86
695 ± 35
3360 ± 190
5440 ± 130
7025 ± 125
8295 ± 85
8260 ± 70
8185 ± 155
8480 ± 60
9410 ± 75
9375 ± 115
10300 ± 75
10490 ± 185
10590 ± 85
11965 ± 140
9505 ± 195
a
All samples are aquatic bryophyte macrofossils. Samples with AA-lab numbers were
submitted to the Arizona Accelerator Mass Spectrometry Laboratory; samples with
CURL-lab numbers were submitted to the University of Colorado INSTAAR Laboratory
for AMS Radiocarbon Preparation and Research.
b
Each calibrated age is the midpoint ± 1/2 the 2σ range calculated using CALIB 5.0.2
(Stuiver et al., 2005).
Figure 2. 14C chronology from Lake CF8 (n = 15; two ages shown as open triangles are
excluded from polynomial fit). 2σ calibrated age ranges are shown for each age. Dashed
gray lines are 95% confidence intervals.
144
Y. Axford et al. / Quaternary Research 71 (2009) 142–149
time do not overwhelm the influence of temperature (e.g., Velle et al.,
2005; Antonsson et al., 2006; Brooks, 2006). For this study, at least 50
whole head capsules per sample were extracted and prepared
following standard methods (e.g., Walker, 2001). Forty-six samples
were analyzed, with sampling concentrated in the early Holocene. July
air temperature inferences were derived from the transfer function
published by Francis et al. (2006), who added 29 calibration sites from
Baffin Island to a training set of 39 sites across eastern Canada (Walker
et al., 1991a; 1997). The weighted-averaging regression model uses
square-root transformed species data and has a root mean squared
error of prediction (RMSEP) of 1.5°C for mean July air temperatures.
Paleotemperatures were calculated using the computer program C2 v
1.4.3 (Juggins, 2003). One of the 19 taxa identified in Lake CF8
sediments (Paracladopelma) is not represented in the training set and
therefore was not used for quantitative temperature inferences. Paracladopelma has a maximum abundance of b2% in the subfossil
samples; thus, N98% of the taxa in subfossil samples are present in the
training set. Canonical correspondence analysis (CCA) was used to test
for analogues to subfossil assemblages in the training set. CCA was
conducted using the statistical software program R v 2.5.1; rare taxa
were downweighted, lake area data square-root transformed, and lake
depth data log-transformed.
Qualitative environmental proxies provide supplementary information about changing paleoenvironments. The organic carbon
content of lake sediments is a function of aquatic and terrestrial
productivity, and influx of inorganic materials. Percent loss-onignition (%LOI) is highly correlated with the total carbon content
(%C) of sediments in Clyde Foreland lakes (Briner et al., 2006). The %
LOI of sediments was measured at 550°C (Heiri et al., 2001) and is
reported as weight-percent C of dry sediment. Biogenic silica (BiSiO2)
in lake sediments primarily comprises diatoms (e.g., Conley, 1988),
and therefore provides a proxy for aquatic primary productivity,
although it is also a function of the flux of other materials to the lake.
BiSiO2 analysis followed Mortlock and Froelich (1989), except for the
use of 10% Na2CO3 solution for BiSiO2 extraction. BiSiO2 concentration
was measured by spectrophotometry and converted to weightpercent SiO2 of dry sediments.
Results
At the onset of postglacial lacustrine sedimentation at Lake CF8, the
cold stenotherm Oliveridia/Hydrobaenus dominated the chironomid
assemblage (Fig. 3 and Fig. 4). Oliveridia/Hydrobaenus was abruptly
replaced by an assemblage dominated by the subtribe Tanytarsina
(including Tanytarsus lugens/Corynocera oliveri type and Micropsectra)
beginning ∼11 cal ka BP, and inferred temperatures rose rapidly to
surpass modern values by ∼10.5 cal ka BP. Early Holocene assemblages
are dominated by Tanytarsina along with Tanypodinae, Sergentia,
Chironomus, and Orthocladiinae (including Cricotopus/Orthocladius,
Corynoneura/Thienemanniella, and Psectrocladius among others). The
presence of relatively thermophilous taxa (e.g., Psectrocladius and the
Tanypodinae), combined with the absence of cold stenotherms
(including cold-tolerant taxa that were present in both the late glacial
and late Holocene) indicate relatively warm temperatures in the early
Holocene. In the eastern Canada training set, for example, Psectrocladium has an optimum (13.7°C; tolerance 3.6°C; Francis et al., 2006) far
above present-day mean July temperature at Lake CF8 (∼ 4.4°C), and
this genus is not found in modern sediments from the lake (personal
observation; Thomas et al., 2008). Accordingly, inferred temperatures
for most of the early Holocene are nearly 5°C (±1.5°C) higher than for
the late Holocene. The %LOI is generally higher for this early Holocene
warm period than for the middle and late Holocene, whereas %BiSiO2
is lowest for the early Holocene (Fig. 3).
Early Holocene warmth was interrupted by two inferred cold
reversals between 9.5 and 8 cal ka BP (Fig. 4). Lower inferred
temperatures during these two periods reflect clear changes in species
assemblages, including the appearance (albeit in small numbers) of
the cold stenotherms Pseudodiamesa (optimum 5.9°C, tolerance 0.9°C)
and Abiskomyia (optimum 6.2°C, tolerance 0.8°C; Francis et al., 2006),
which were otherwise absent throughout the early Holocene after the
Figure 3. Figure showing percentages of all chironomid taxa, head capsule concentrations, chironomid-inferred July air temperatures, percent loss-on-ignition, and percent biogenic
silica versus depth and age. Taxa on the left side of the diagram are ordered by temperature optimum from coldest to warmest (optima of remaining taxa are unknown). Tanytarsus
lugens/Corynocera oliveri, Micropsectra, Paratanytarsus, and Tanytarsina undiff are lumped as “subtribe Tanytarsina” in the training set of Francis et al. (2006), and therefore also in
temperature modeling. Error bars on temperature inferences are the model RMSEP.
Y. Axford et al. / Quaternary Research 71 (2009) 142–149
145
Figure 4. Percentages of selected chironomid taxa, number of cold indicator taxa present in each sample (indicator taxa are Oliveridia/Hydrobaenus, Pseudodiamesa, and Abiskomyia)
and chironomid-inferred July air temperatures. Unfilled white curves on Pseudodiamesa, Abiskomyia, and Paracladopelma diagrams are 5× exaggerated. Horizontal gray bars highlight
inferred cold reversals.
onset of warmth (Fig. 4). Heterotrissocladius (optimum 8.7°C, tolerance
3.3°C; Francis et al., 2006), which was rare during the early Holocene
but abundant in the late Holocene, spiked in abundance during the
cold events. Paracladopelma appeared during and immediately after
the inferred reversals. The earlier reversal corresponds with a
bryophyte-rich layer from 86–84 cm depth and correspondingly
high %LOI (Fig. 3). The two temperature reversals were of similar
amplitude, and both lasted 250 yr or less. The two cold periods were
separated by several centuries of near-peak inferred warmth.
Following a return to warm conditions after the latter cold reversal,
the site began to cool ∼ 7.8 cal ka BP. Sedimentation rates decreased
slightly in the late Holocene, a period characterized by the
disappearance of T. lugens/C. oliveri type, and greater percentages of
taxa with cold affinities, including Abiskomyia, Oliveridia/Hydrobaenus,
and Pseudodiamesa. Tanypodinae disappeared ∼7.5 cal ka BP and did
not reappear. The cold stenotherms Oliveridia/Hydrobaenus and
Pseudodiamesa became increasingly abundant after ∼5 cal ka BP,
with peak percentages in the last millennium, indicating intensified
Neoglacial cooling.
Because the coldest site in the training set has a mean July air
temperature of 5.0°C (Francis et al., 2006), the transfer-function
calibration data do not contain adequate analogs for colder-thanpresent times when cold stenotherms were more abundant than
today at our relatively cold study site (mean July air temperature
4.4°C). Therefore, the obvious chironomid assemblage changes in the
late Holocene do not result in correspondingly lower temperature
inferences (Fig. 3 and Fig. 4). For this reason, the amplitude of late
Holocene temperature variation is presumably underestimated, and
we restrict our discussion here to events of the early Holocene, when
temperatures were warmer than present and well within the range of
July air temperatures represented by the training set (5.0 to 19.0°C).
Thomas et al. (2008) describe a temporally detailed chironomid
record for the last two thousand years at this site, including rapid
assemblage changes that occurred in the past century.
Discussion
Holocene thermal maximum
Chironomid-based reconstructions indicate that early Holocene
summers at Lake CF8 and nearby Lake CF3 (22 km to the southeast;
Briner et al., 2006) were nearly 5°C (±1.5°C) warmer than present,
presumably in response to a positive anomaly in summer insolation
forcing (Berger and Loutre, 1991; Fig. 5A). This magnitude of
reconstructed early Holocene warmth far exceeds hemispheric
averages, even for high-latitude sites. Kaufman et al. (2004) reviewed
varied peak Holocene temperatures (mostly summer temperatures
estimated from pollen) ranging from 1 to 3°C higher than present
around the Western Hemisphere Arctic. Borehole measurements
indicate that peak annually integrated Holocene temperatures on the
central and southwestern Greenland Ice Sheet were ∼ 2°C higher than
present (Cuffey and Clow, 1997; Dahl-Jensen et al., 1998), suggesting
either less summer warming over Greenland relative to Baffin Island,
or greatly enhanced seasonality relative to present.
Like all proxy-based reconstructions, chironomid-inferred temperatures come with caveats: for example, if modern calibration
datasets do not include appropriate analogues for past chironomid
assemblages and past climate (e.g., Williams and Jackson, 2007),
transfer functions will yield spurious temperature reconstructions.
We used CCA to test whether our calibration dataset contains useful
analogues for early Holocene assemblages at Lake CF8. In Figure 6, all
of the downcore samples from Lake CF8 are plotted passively on a
CCA biplot of surface-sediment samples from the calibration (training
set) sites (Walker et al., 1997; Francis et al., 2006). The Holocene
samples appear to have good analogues in the calibration dataset:
Mid- to late Holocene samples have their closest analogues on
modern-day Devon and Baffin islands. Samples from the Holocene
Thermal Maximum plot in a cluster that overlaps with warmer
calibration sites farther south in Atlantic Canada. The two late-glacial
146
Y. Axford et al. / Quaternary Research 71 (2009) 142–149
Figure 5. (a) Holocene July air temperatures at Lake CF8 compared with July insolation (Berger and Loutre, 1991) and melt layers in the Agassiz Ice Cap (Fisher and Koerner, 2003). (b)
Early Holocene July air temperatures at Lake CF8 compared with Ammersee ostracode δ18O (‰ PDB; von Grafenstein et al., 1999), NGRIP δ18O (‰ VSMOW; NGRIP Project Members,
2004; GICC05 chronology from Rasmussen et al., 2006 and Vinther et al., 2006), GISP2 K+ ions (ppb; Mayewski et al., 1997), and Dongge Cave speleothem δ18O (‰ VPDB; Dykoski et
al., 2005). The World Data Center for Paleoclimatology in Boulder, Colorado, provided access to archived data.
samples have no analogues in the modern data set, but their strongly
negative Axis 1 scores are consistent with very cold air and water
temperatures.
Another potential problem with quantitative temperature reconstructions is that intercorrelations among environmental variables
known to influence chironomid species distributions (e.g.,
Figure 6. CCA site scores for chironomid assemblages in surface-sediment samples of the training set, shown as black circles (training set sites and samples are described by Walker et
al., 1997, and Francis et al., 2006). Downcore samples from Lake CF8 are plotted passively and shown as open squares (late-glacial samples), open triangles (samples recording peak
inferred temperatures of the early Holocene), and gray triangles (samples from the mid- to late Holocene and the early Holocene cold reversals). Vectors represent environmental
data from the training set, including summer lake-water temperature, July air temperature, lake depth, and lake surface area.
Y. Axford et al. / Quaternary Research 71 (2009) 142–149
intercorrelations among air and water temperatures, primary productivity, and food availability) may tend to amplify the effects of warming
on chironomid assemblages. However, to the extent that such
intercorrelations have persisted through time, this effect is implicitly
factored into modern calibration data. An additional complication is
that at some high-latitude sites, midge assemblage shifts on millennial
time scales through the Holocene appear to primarily reflect the effects
of post-glacial soil development and vegetation succession, and
attendant changes in lake depth, water chemistry, and trophic status,
rather than direct responses to temperature change (e.g., Rosen et al.,
2003; Velle et al., 2005). In these cases, multi-proxy comparisons
reveal divergent trends in chironomid-inferred temperatures versus
other temperature-dependent proxies (e.g., pollen).
The timing and direction of chironomid-inferred temperature
changes at Lake CF8 are consistent with results from several other
studies in the region, such as inferred primary productivity in many
northern Baffin Island lakes and chironomid-inferred temperatures at
neighboring Lake CF3, peaked between 10 and 8 cal ka BP (Miller et al.,
2005; Briner et al., 2006; Micheluti et al., in press). The first millennia
of the Holocene also saw peak melt on the Agassiz Ice Cap on
Ellesmere Island (Fig. 1 and Fig. 5; Fisher and Koerner, 2003). Knudsen
et al. (2008) infer maximum Atlantic-water influence in northernmost
Baffin Bay in the earliest part of the Holocene, with subsequent
increased influence of sea ice after ∼7.3 cal ka BP. However, the
inferred timing of maximum Holocene warmth in Baffin Bay and the
Arctic channels varies between sites and proxies, with some records
suggesting that peak warmth within Baffin Bay occurred millennia
after peak warmth at Lake CF8 (Dyke et al., 1996a,b; Levac et al., 2001).
It has been proposed that early Holocene warming in parts of the
North Atlantic region lagged summer insolation forcing due to the
cooling influence of the residual Laurentide ice sheet (Kaufman et al.,
2004; Kaplan and Wolfe, 2006), but results from Lake CF8 suggest that
the terrestrial environment of northern Baffin Island responded
rapidly and dramatically to insolation forcing.
Additional studies using quantitative temperature proxies should
clarify whether chironomid assemblages are providing a realistic
estimate of the magnitude and timing of early Holocene warmth at
Lakes CF3 and CF8. Several factors may indeed have combined to cause
exceptional warming over the eastern Canadian Arctic in response to
enhanced early Holocene insolation forcing: for example, amplification of warming by changes in the duration of terrestrial snowcover
(e.g., Chapin et al., 2005) and possibly sea-ice albedo feedbacks related
to waning sea ice cover over Baffin Bay and the channels of the Arctic
Archipelago (Dyke et al., 1996b).
Abrupt cold reversals in the early Holocene
Prior Holocene temperature reconstructions on Baffin Island (e.g.,
at Lake CF3, Fog Lake, and Brother of Fog Lake; Briner et al., 2006;
Francis et al., 2006; Fréchette et al., 2006) were pioneering studies
done at millennial-scale resolution and did not resolve sub-millennial
climate shifts. A new finding from the more resolved Lake CF8
paleotemperature record is the pair of abrupt cold events that
occurred during peak early Holocene warmth. Abundances of cold
stenotherms were low during the reversals, but their presence at
these times was unique to the early Holocene (Fig. 4). Pseudodiamesa
and Abiskomyia are present in the surface sediments of only the
coldest lakes in Canada and Scandinavia today (Olander et al., 1999;
Francis et al., 2006). Notably, Paracladopelma also appears during the
reversals. The temperature optimum of this genus in eastern Canada is
unknown, but in west Greenland Paracladopelma is most abundant in
the coldest oligotrophic lakes, and it appears only in the highestelevation lakes in the European Alps (Lotter et al., 1997, 1999;
Brodersen and Anderson, 2002).
Although our age model suggests that the latter cold event at Lake
CF8 dates to ∼ 8.5 to 8.6 cal ka BP, the age model has an uncertainty
147
of ± 310 yr for this time period (Fig. 2), and correlation with the
conspicuous cold reversal documented elsewhere ∼8.2 cal ka BP (e.g.,
Alley et al., 1997; Alley and Ágústdóttir, 2005) is possible. The advance
of a local ice cap and decreased lake productivity ∼ 8 cal ka BP were
recently reported from southern Baffin Island (Miller et al., 2005), and
the abrupt decline of birch pollen in north-central Canada (Seppä et al.,
2003) provides supporting evidence that Arctic Canada cooled ∼ 8.2 cal
ka BP. However, the age of the latter reversal at Lake CF8 is not tightly
constrained and correlation with the “8.2 ka event” is tentative.
The earlier reversal dates to ∼ 9.2 to 9.3 cal ka BP. We speculate that
cooling at Lake CF8 at this time was part of a widespread climate
perturbation that affected the North Atlantic region and parts of Asia.
Prior paleoclimate studies have documented abrupt climate changes
around the North Atlantic region and farther afield between 9.5 and
9.0 cal ka BP (Fig. 5B), including cooling over central Greenland and
the DYE-3 site in southern Greenland (e.g., NGRIP, 2004; Vinther et al.,
2006), changes in precipitation isotopes over the British Isles and
central Europe (von Grafenstein et al., 1999; McDermott et al., 2001),
and weakened monsoon precipitation inferred from speleothems in
southern China (Dykoski et al., 2005) and Oman (Fleitmann et al.,
2007). Fleitmann et al. (2008) review evidence for a climate
perturbation that affected much of the Northern Hemisphere at this
time, and they hypothesize that it may have been caused by a
meltwater pulse into the North Atlantic. Perturbations in some
paleoclimate proxies at this time show comparable amplitude to
that of the 8.2 ka event, although at many sites the 8.2 ka event is of
unique amplitude for the Holocene.
End moraines across Baffin Island record advances of Laurentide
outlet glaciers and local mountain glaciers between ∼9.5 and 8 cal ka
BP (Cockburn substage sensu lato; e.g., Andrews and Ives, 1978; Dyke
and Hooper, 2001; Miller et al., 2005). The cause of these glacier
advances has been enigmatic, but their timing is broadly coincident
with the timing of abrupt cold events documented at Lake CF8 and
farther afield. If these cold reversals did indeed cause advances of the
Laurentide ice sheet, cooling must have occurred over large parts of
Canada.
The occurrence of two separate cold events of similar magnitude
and duration indicates that the well-known 8.2 ka event, whether or
not it affected Lake CF8, was not uniquely significant at this site. An
event as catastrophic as the final draining of glacial lakes Agassiz and
Ojibway—a postulated cause of the 8.2 ka event (e.g., Barber et al.,
1999)—may not be required to trigger widespread perturbations of
interglacial climate. Evidence from a growing number of sites for highamplitude abrupt, widespread climate shifts in the early Holocene
illustrates the sensitivity of the climate system to perturbations, even
during periods of warmth and enhanced radiative forcing in the
Northern Hemisphere.
Conclusions
Because enhanced summer insolation forcing in the early Holocene
was long-lived and hemispherically symmetric, this time period
provides a useful (albeit imperfect) analog for future greenhouse
warming. Extreme early Holocene warmth on Baffin Island, if
confirmed by future studies and additional proxies, may foretell
strong future warming in the region. Climate feedbacks, along with
the spatially heterogeneous nature of early Holocene warmth across
the Arctic, present challenges for predicting the pattern of future
warming and its impacts. Fortunately, a growing database of
quantitative paleoclimate records is elucidating the pattern of early
Holocene warmth across the Arctic and improving our understanding
of the factors that modulate local and regional responses to radiative
forcing.
This study adds to mounting evidence for widespread abrupt
climate change ∼9.2 cal ka BP, and for climatic variability during the
Holocene in general. At some sites the amplitude of the “9.2 ka event”
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Y. Axford et al. / Quaternary Research 71 (2009) 142–149
matches or exceeds that of the 8.2 ka event, indicating that major
Holocene climate perturbations were not limited to 8.2 cal ka BP.
Given lingering uncertainty regarding the probability of “climate
surprises” in Earth's warm greenhouse future (NRC, 2002; IPCC, 2007),
and about the potential climatic fallout of changing freshwater inputs
to the North Atlantic Ocean (e.g., Curry and Mauritzen, 2005; Zickfeld
et al., 2007), abrupt climate shifts in the early Holocene remain
important targets for research. These events may especially shed light
on the role of freshwater forcing on North Atlantic Ocean circulation,
as outbursts of freshwater from the Laurentide ice sheet during this
time are hypothesized triggers for abrupt early Holocene climate
perturbations.
Acknowledgments
E. Farris and E. Thomas helped with chironomid preparations. R.
Coulthard, J. Hainnu, J. Qillaq and A. Wolfe assisted in the field. VECO
Polar Resources provided logistical support of field work, and the
Nunavut Research Institute (Nunavummi Qaujisaqtulirijikkut) helped
with logistics in Iqaluit and granted access to the field site. Gratitude is
extended to the people of Clyde River, Nunavut, for their hospitality
and help with field work. J. Turnbull, C. Wolak (INSTAAR) and M.
deMartino (University of Arizona) prepared 14C samples. Thanks to D.
Kaufman and C. Schiff (Northern Arizona University) and F.S. Hu and L.
Roschen (University of Illinois) for BiSiO2 measurements. C. Schiff also
provided advice about age model development. This research was
funded by Geological Society of America graduate student grants, the
U.S. National Science Foundation, and University at Buffalo faculty
start-up funds. A. Dyke, M. Kaplan, J. Knox, D. Porinchu and two
anonymous reviewers provided helpful feedback on this manuscript.
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